A light-emitting device utilizing an emission from an electroluminescent element (EL: Electro Luminescence) has attracted attention as a display device or a lighting device, and the development has been advanced.
In development of a light-emitting device, it is an object of the present invention to control decrease over time in emission brightness of a light-emitting element and to extend the lifetime thereof.
Therefore, the development of a light emitting material whose lifetime can be longer and the development of a sealing technique by which the degradation of the light-emitting material can be decreased have been carried out. In addition, as described in patent document 1, for example, it has been tried to extend a lifetime of a light-emitting layer formed of an inorganic compound by heat treatment at from 400° C. to 500° C. after an oxidation.
However, oxidation as shown in the patent document 1 needs heat treatment of 250° C. to 450° C. It is thought that such a heat treatment is improper for a light-emitting element having an organic material for a light emitting layer which has lower decomposition temperature and inferior heat resistance in comparison with the inorganic compound.
Thus, a new means for obtaining long lifetime of the light-emitting element using an organic compound as a light-emitting layer is required to be developed.
[Patent Document 1]
Japanese Patent Laid-Open No. 7-45367